Adds better documentation to LOCAL_DOMAIN and LOCAL_HTTPS (#3149)

Fixes #2254

.env.production.sample

@@ -10,7 +10,9 @@ DB_PASS=
 DB_PORT=5432
 
 # Federation
-LOCAL_DOMAIN=example.com
+# Note: Changing LOCAL_DOMAIN or LOCAL_HTTPS at a later time will cause unwanted side effects.
+# LOCAL_DOMAIN should *NOT* contain the protocol part of the domain e.g https://example.com.
+LOCAL_DOMAIN=example.com 
 LOCAL_HTTPS=true
 
 # Use this only if you need to run mastodon on a different domain than the one used for federation.